Disk drives typically employ electromagnetic read/write heads to store and recover information from the recording surfaces of rotating disks. The disks generally include a high coercivity magnetic storage layer at each planar surface. Data is normally stored along concentric data tracks in the magnetic storage layer. For disk drives having more than one data recording surface, the set of tracks at a common radial location is referred to as a cylinder. Head positioning relative to a track or cylinder is usually performed using a closed-loop servo scheme, so that the head(s) can be precisely positioned and can track-follow. Servo positioning can be performed with respect to servo data recorded solely on a dedicated disk surface (i.e., "dedicated-servo") or with respect to servo information embedded in every track of every recording surface (i.e., "embedded-servo"). The latter servo scheme tends to provide more accurate head positioning and in the case of a disk drive having few disks, an embedded-servo scheme will tend to use a smaller percentage of the potential data storage area for servo overhead. Accordingly, disk drives incorporating embedded-servo schemes currently dominate the market.
In order for an embedded servo drive to operate properly, the servo information that ultimately defines the position of the data tracks must be written with great accuracy. Typically, the servo information is written on each surface as a radially extending set of spokes or "wedges." The portion of a servo wedge at a particular track location typically contains a sync field, an index mark, a gray coded track number, and two or more fine-position, offset bursts configured in an echelon across the track. Other formats and servo patterns are also employed within embedded servo disk drives. Head positioning relative to a track center can be determined and corrected, if necessary, by reading and noting the respective amplitudes and timings of these latter offset bursts.
Traditionally, the machine used to write the embedded servo information is called a servowriter. A servowriter typically includes a large, massive granite base to minimize the effects of vibration, precision fixtures to hold the target disk drive, a precision laser interferometer based actuator arm positioning mechanism to position the arms radially with respect to the axis of rotation of the disks in the drive, and an external clock head to position the servo wedges in time (i.e., at a particular angular offset with respect to a fixed radial line segment on the disk). See, e.g., FIG. 1 of commonly assigned U.S. Pat. No. 4,920,442. Conventional servowriters are expensive.
Typically, a drive to be servowritten must be servowritten with its cover removed or with at least two external openings to permit the insertion of the clock head and the arm positioning mechanism when the drive is mounted on the servowriter. The requisite openings or holes give rise to several problems because the holes adversely affect the mechanical integrity of the disk drive's base casting or baseplate, thus causing stiffness and resonance related concerns. Also, the cutouts provide an ingress for contaminants which can easily ruin a disk drive. Servowriting with the drive's cover removed amplifies these problems. The entry of contaminants not only increases tribology related problems, but more importantly, because the spacing between a head and disk is typically not more than a few microinches, the entry of even microscopic particulate contaminates can lead to a catastrophic failure such as a head-to-disk crash. Accordingly, servowriters are usually installed in a clean room in order to avoid contaminating the target drive.
In a prior art servowriting process, a target drive to be servowritten is mounted on a fixture that is rigidly attached to the stabilizing granite base. The drive is not fully assembled at this stage of manufacture, hence the partially assembled drive is sometimes referred to as a head disk assembly (HDA), which usually includes a baseplate with the spindle motor, actuator assembly, heads, disks and some of the electronic circuitry installed. A clock head is inserted into a hole in the HDA and a precision fixture is attached to or inserted in the HDA to provide feedback to a laser sensor mechanism used to physically position the recording head(s) to write the servo wedges at the appropriate radial locations during the servowrite operation. Because both the arm positioning mechanism and the clock head need to be physically inserted into the drive, the required HDA openings necessitate the use of a clean room environment for the servowriting operation. The physical insertion of a clock head into a disk drive HDA also creates a potential for clock head-to-disk crash, so conventional servowriting is a potentially risky operation for the target drive. The clock head is used to write a clock track, using one of many well known methods, near the outer edge of one of the disk surfaces. Then, a phase locked loop is locked to the clock track. The phased locked loop clocks a counter that is reset once per revolution. As the spindle motor speeds up or slows down, the phase locked loop will accurately track circumferential position at all times. Having thus established an accurate determination of the disk phase, the servowriter then uses the laser-interferometer controlled arm positioning mechanism to accurately position the heads radially with respect to the axis of rotation of the disk(s) and proceeds to dump (i.e., write) the servo data.
Due to the incorporation of large granite structures, laser based sensors, and high precision fixtures, present servowriters are extremely large and expensive. As track densities increase, the servowrite time also increases, which can create a bottleneck in the disk drive manufacturing process at the servowriter station. Therefore, the time that a disk drive HDA is being servowritten is expensive, because that time limits the achievable manufacturing throughput. Because laser-based servowriters typically require the use of a clean room dating normal operation to avoid contamination of the target disk drive, a hitherto unsolved need exists for an improved, lower cost, non-invasive servowriter and method which overcomes prior art limitations including, inter alia, high costs of procurement and operation.